Verizon Wireless stands ready to hand back millions of dollars to customers who were overcharged on their mobile phone bills due to built-in phone software that accesses the network on its own.

Over the next two months, Verizon will notify 15 million customers that they were mistakenly overcharged and that they will be given credits to their accounts. Many of the credits will be refund checks for $2 to $6, though some will receive larger credits or refunds, Verizon said in a statement.

“As we reviewed customer accounts, we discovered that over the past several years approximately 15 million customers who did not have data plans were billed for data sessions on their phones that they did not initiate,” the company said.